CELL SURFACE ANTIGENS IN EARLY MAMMALIAN DEVELOPMENT The cell surface is known to be of direct functional significance in a number of developmental systems. Communication between cells is mediated at the cell membrane and serves as a basis for the control of embryogenesis. During mammalian development, surface constituents are likely to be of importance as early as the preimplantation period. However, specific developmental roles have not as yet been ascribed to any specific cell surface molecules expressed by preimplantation mammalian embryos. The research proposed here is designed to extend the characterization of specific cell surface antigens detected by the A-BL-2 anti-blatocyst serum which are expressed in a stage-specific manner during mouse preimplantation development. Experiments will assess (1) the antigenic specificity of A-BL-2 antibodies, (2) the functional role of A-BL-2 detected antigens during preimplantation development, (3) the biology of their synthesis and cell surface expression, and (4) their biochemical characteristics and molecular associations. A possible functional role for A-BL-2 detected surface antigens will be investigated by immuno-complexing these molecules with A-BL-2 antibody probes (serum, IgG and Fab) and monitoring (a) major morphologically identifiable developmental events, (b) embryo ultrastructure, (c) biochemical functions, and (d) surface antigen density and distribution. The synthesis and cell surface expression of A-BL-2 detected antigens will be investigated at the ultrastructural level by immunocytochemical localization. Biochemical characterizations will assess (a) relationships between A-BL-2 antigen proteins and their associations with viral proteins, (b) the molecular associations of A-BL-2 antigens at the embryonic cell surface and the possible existence of A-BL-2 receptor molecules, and (c) the requirement for the paternal genome and synthetic functions of the embryo in the synthesis and cell surface expression of A-BL-2 detected antigens. The results of these studies are expected to contribute to the basic understanding of cell surface function and its participation in mammalian development, in addition to having potential significance in the area of reproductive control.